The present application relates to a new efficient, economical, nonpolluting cyclic process for producing boric acid by in situ solution mining of a subterranean deposit containing a borate ore which preferably is flooded or located below the water table.
The conventional method of producing boric acid is to react already mined borate-containing ore with sulfuric acid. In the United States of America the mined ore generally contains high concentrations of the mineral kernite, a sodium borate: Na.sub.2 B.sub.4 O.sub.7.4H.sub.2 0. The reaction with sulfuric acid proceeds according to the following equation: EQU Na.sub.2 O.2B.sub.2 O.sub.3.4H.sub.2 O+H.sub.2 SO.sub.4 +H.sub.2 O Na.sub.2 SO.sub.4 +4H.sub.3 BO.sub.3
The boric acid is separated from the more readily soluble sodium sulfate by selective crystallization. See, e.g., U.S. Pat. Nos. 3,917,801; 3,953,580; Japanese No. 7,002,650; U.S. Pat. Nos. 4,270,944; 1,944,598, 3,103,412; and 2,855,276.
In Europe, however, the generally available mineral is colemanite, Ca.sub.2 B.sub.6 O.sub.11.5H.sub.2 O, e.g., imported from Turkey. In this case, the reaction with acid proceeds according to the equation: EQU Ca.sub.2 B.sub.6 O.sub.11.5H.sub.2 O+2H.sub.2 SO.sub.4 +6H.sub.2 O 6H.sub.3 BO.sub.3 +2CaSO.sub.4.2H.sub.2 O
The mineral is first crushed ad ground to a fine particle size and then leached with sulfuric acid in agitated reactors at elevated temperatures. Separation is relatively easy since the boric acid goes into solution and is separated from the sparingly soluble gypsum and other insoluble components of the ore by filtration. Boric acid is then recovered from the filtrate by crystallization.
Boric acid may also be produced by alkaline leaching of borate ores. This practice is well known and is described e.g., in: German No. 2,020,570; U.S. Pat. No. 3,829,553; German No. 2,608,597; G.B. Nos. 158,992; 1,379,098; U.S. Pat. Nos. 3,218,120; 4,022,871; G.B. No. 1,297,743; and Romanian No. 52,241.
In the past, hydrochloric acid has also been used to solubilize mined borate ores. U.S. Pat. No. 1,308,577 uses HCl to produce boric acid from a borate salt (borax), and U.S. Pat. No. 2,855,276 digests already mined crude colemanite ore using hydrochloric acid. Advantageous use of the classic salting-out effect is also discussed in this reference, and in others in conjunction with the mining of other borate ores, e.g., in U.S. Pat. No. 1,927,013.
Other lixiviants are also known in the mining of borate ores such as colemanite, e.g., ammonium sulfate as taught in U.S. Pat. No. 3,103,412 wherein the lixiviant is regenerated and recycled.
However, all of these conventional techniques have serious disadvantages. For example, all require the movement of large tonnages of ore and waste rock and all require crushing and grading of the ore. In addition, these conventional techniques tend to be capital cost intensive with respect to plant and equipment.
In situ solution mining methods which eliminate some of these disadvantages have been applied to the mining of other types of elements. For example, this method has proved to be very effective for the recovery of uranium from sandstone and other deposits which have the requisite degree of permeability. The latter property of the deposit is critical in controlling the rate of recovery of mineral values by solution mining techniques. The problem is especially severe when the leaching reaction produces insoluble byproducts stemming from either the main ore or other components of the deposit. Because of these problems, the applicability of solution mining to other types of ores has been limited. See, e.g., U.S. Pat. No. 4,103,963; USBM - IC 8777, 1978 ("Uranium In Situ Leach Mining in the United States"); Yan, J. Pet. Tech. 32(1980) 2068; and U.S. Pat. No. 3,574,599, for general discussions of the application of the solution mining technique to ores, usually uranium ores, and, for a discussion of the importance of maintaining high permeability, i.e., avoiding deposit blockage, usually by addition of a chemical inhibitor.
The same difficulty of blockage has also been encountered in other types of mining operations, e.g., in petroleum mining. Various methods of avoiding such blockages have been developed, including the addition of HCl to subterranean wells to dissolve deposits as disclosed, e.g., in U.S. Pat. Nos. 2,356,205; 3,353,603; 3,708,014; 1,891,667; 1,969,230; and 556,669.
In conjunction with solution mining, solar evaporation ponds have sometimes been employed to recover the desired salt from the produced brine (U.S. Pat. Nos. 4,072,472 and 3,966,541).